In orchard areas, in the Spring of the year, temperature inversions are common, and can cause damage to the fruit buds and blossoms. During a temperature inversion, the air temperature at the level of the trees is colder than the air temperature above the orchard. In some orchard areas where there is no air drainage, the still air can become particularly cold.
Various methods are used to protect the buds and blossoms against freezing. A fine water spray system can assure, up to a point, that the buds and blossoms remain at the freezing temperature of water, as the water is continually at the latent heat of fusion. A great many orchardists use wind machines, which are large motor-driven propellers mounted on towers above treetop level. Of course, ground-level heaters are also used.
Ideally, it is preferable to pull the warm air down from above the treetop level and distribute it at or below the leafy portions of the trees.